1. Field of the Invention
The present invention relates to a holey fiber and a method of manufacturing the same.
2. Description of the Related Art
A holey fiber is an optical fiber having air holes arranged in a periodic manner in a cladding region. The cladding region surrounds a core region. The cladding region has reduced average refractive index because of the presence of the air holes so that a light passes almost entirely through the core region because of the total reflection of the light.
Because the refractive index of holey fibers can be controlled by controlling various parameters of the air holes, the holey fibers can realize unique properties that can not be realized in the other optical fibers such as endlessly single mode (ESM) and anomalous dispersion at a short wavelength. The ESM means that a cut-off wavelength is not present and a light is transmitted in a single mode at all wavelengths. With the ESM, it is possible to realize an optical transmission at a high transmission speed over a broadband. For example, a result of an experiment of a dispersion-managed soliton transmission at a transmission speed of 10 Gb/s by forming an optical path of 100 kilometers by combining the holey fiber and a dispersion compensating optical fiber is disclosed in K. Kurokawa, et al., “Penalty-Free Dispersion-Managed Soliton Transmission over 100 km Low Loss PCF”, Proc. OFC PDP21 (2005).
Holey fibers are manufactured by drawing a preform. The preform is made of silica glass and holes are formed in the preform by stack-and-draw method, drill method, or sol-gel method. Shapes of the air holes can disadvantageously deform during the drawing process. A technology for preventing deformation of shapes of air holes during the drawing process is disclosed in, for example, Japanese Patent Application Laid-open No. 2006-83003 and Jonathan C. Knight, “Photonics crystal fibers”, NATURE Aug. 14, 2003, Vol. 424, pp. 847-851. Specifically, the preform is drawn while precisely controlling the pressure of inert gas.
In holey fibers, the cladding region has reduced average refractive index because of the presence of the air holes so that a light is almost entirely confined in the core region. Generally, upon forming the air holes around the core region in layers, three or more layers are necessary. Furthermore, upon arranging the air holes in triangular lattices in such layers, a large number of the air holes are necessary. Specifically, the number of air holes increases as the number of layers increases. For example, as much as 36 air holes are necessary in a three-layer structure, 60 in a four-layer structure, and 90 in a five-layer structure. The characteristics of the holey fiber depend on the number of the layers, inner diameters of the air holes, and air hole pitches of the air holes.
However, if inner diameters and air hole pitches of all the air holes are to be controlled precisely for manufacturing the holey fiber with desired characteristics, operational load and processing time increase. This reduces the productivity.